Download 8. Prime Factorization and Primary Decompositions

70
Andreas Gathmann
8.
Prime Factorization and Primary Decompositions
When it comes to actual computations, Euclidean domains (or more generally principal ideal domains) are probably the “nicest” rings that are not fields. One of the main reasons for this is that
their elements admit a unique prime factorization [G1, Proposition 11.9]. This allows for an easy
computation of many concepts in commutative algebra, such as e. g. the operations on ideals in
Example 1.4.
Unfortunately however, such rings are rather rare in practice, with the integers Z and the polynomial
ring K[x] over a field K being the most prominent examples. So we now want to study in this chapter
if there are more general rings that allow a prime factorization of their elements, and what we can
use as a substitute in rings that do not admit such a factorization.
More precisely, given an element a 6= 0 in an integral domain R which is not a unit we ask if we can
write a = p1 · · · · · pn for some n ∈ N>0 and p1 , . . . , pn ∈ R such that:
• The pi for i = 1, . . . , n are irreducible or prime — recall that in a principal ideal domain these
two notions are equivalent, but in a general integral domain we only know that every prime
element is irreducible [G1, Lemma 11.3 and Proposition 11.5].
• The decomposition is unique up to permutation and multiplication with units, i. e. if we also
have a = q1 · · · · · qm with q1 , . . . , qm irreducible resp. prime, then m = n and there are units
c1 , . . . , cn ∈ R and a permutation σ ∈ Sn such that qi = ci pσ (i) for all i = 1, . . . , n.
Let us first discuss the precise relation between the different variants of these conditions.
Proposition and Definition 8.1 (Unique factorization domains). For an integral domain R the following statements are equivalent:
(a) Every non-zero non-unit of R is a product of prime elements.
(b) Every non-zero non-unit of R is a product of irreducible elements, and this decomposition is
unique up to permutation and multiplication with units.
(c) Every non-zero non-unit of R is a product of irreducible elements, and every irreducible
element is prime.
13
If these conditions hold, R is called factorial or a unique factorization domain (short: UFD).
Proof.
(a) ⇒ (b): Let a ∈ R be a non-zero non-unit. By assumption we know that a = p1 · · · · · pn for
some prime elements p1 , . . . , pn . As prime elements are irreducible [G1, Lemma 11.3], we
therefore also have a decomposition into irreducible elements.
Moreover, let a = q1 · · · · · qm be another decomposition into irreducible elements. Then p1
divides a = q1 · · · · · qm , and as p1 is prime this means that p1 divides one of these factors,
without loss of generality p1 | q1 . Hence q1 = c p1 for some c ∈ R. But q1 is irreducible and
p1 is not a unit, so c must be a unit. This means that p1 and q1 agree up to multiplication
with a unit. Canceling p1 in the equation p1 · · · · · pn = q1 · · · · · qm by p1 now yields
p2 · · · · · pn = c · q2 · · · · · qm , and continuing with this equation in the same way for p2 , . . . , pn
gives the desired uniqueness statement.
(b) ⇒ (c): Let p ∈ R be irreducible, we have to show that p is prime. So let p | ab, i. e. ab = pc for
some c ∈ R. By assumption we can write all these four elements as products of irreducible
elements and thus obtain an equation
a1 · · · · · an · b1 · · · · · bm = p · c1 · · · · · cr .
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Prime Factorization and Primary Decompositions
71
But by the uniqueness assumption, the factor p on the right must up to a unit be one of the
a1 , . . . , an or b1 , . . . , bm , which implies that p | a or p | b.
(c) ⇒ (a) is trivial.
Remark 8.2. In Proposition 8.1, the assumption in (b) and (c) that every non-zero non-unit can be
written as a product of irreducible elements is a very weak one: it is satisfied e. g. in every Noetherian
domain by Exercise 7.22 (a). The other conditions are much stronger, as we will see in Examples
8.3 (b) and 8.7.
Example 8.3. The following two examples are already known from the “Algebraic Structures” class:
(a) As mentioned above, principal ideal domains (so in particular Z and univariate polynomial
rings over a field) are unique factorization domains [G1, Proposition 11.9].
√
√
√
5
i]
⊂
C
the
element
2
obviously
divides
6
=
(1
+
5
i)(1
−
5 i), but
(b) In the ring R
=
Z[
√
√
neither 1 + 5 i nor 1 − 5 i. Hence 2 is not prime. But one can show that 2 is irreducible
in R,√
and thus R
√ is not a unique factorization domain [G1, Example 11.4]. In fact, 2 · 3 =
(1 + 5 i)(1 − 5 i) are two decompositions of the same number 6 that do not agree up to
permutation and units.
It follows by (a) √
that R cannot be a principal ideal domain. One can also check this directly:
the ideal (2, 1 + 5 i) is not principal [G1, Exercise 10.44]. In fact, we will see in Example
13.28 that up to multiplication with a constant this is the only non-principal ideal in R.
We will see in Example 13.8 however that R admits a “unique prime factorization” for ideals
(instead of for elements) — a property that holds more generally in so-called Dedekind
domains that we will study in Chapter 13.
Remark 8.4. The most important feature of the unique factorization property is that it is preserved
when passing from a domain R to the polynomial ring R[x]. Often this is already shown in the
“Introduction to Algebra” class, and so we will only sketch the proof of this statement here. It relies
on the well-known Lemma of Gauß stating that an irreducible polynomial over Z (or more generally
over a unique factorization domain R) remains irreducible when considered as a polynomial over Q
(resp. the quotient field K = Quot R). More precisely, if f ∈ R[x] is reducible in K[x] and factors as
f = gh with non-constant g, h ∈ K[x], then there is an element c ∈ K\{0} such that cg and hc lie in
R[x], and so f = (cg) · hc is already reducible in R[x] [G3, Proposition 3.2 and Remark 3.3].
Proposition 8.5. If R is a unique factorization domain, then so is R[x].
Sketch proof. We will check condition (c) of Definition 8.1 for R[x].
Let f ∈ R[x], and let c be a greatest common divisor of all coefficients of f . Then f = c f 0 for
some polynomial f 0 ∈ R[x] with coefficients whose greatest common divisor is 1, so that no constant
polynomial which is not a unit can divide f 0 . Now split f 0 into factors until all of them are irreducible
— this process has to stop for degree reasons as we have just seen that the degree of each factor must
be at least 1. But c can also be written as a product of irreducible elements since R is a unique
factorization domain, and so f = c f 0 is a product of irreducible elements as well.
Next assume that f is irreducible, in particular we may assume that c = 1. We have to show that f
is also prime. So let f divide gh for some g, h ∈ R[x]. If we denote by K the quotient field of R, then
f is also irreducible in K[x] by Remark 8.4, hence prime in K[x] by Example 8.3 (a), and so without
loss of generality f | g in K[x]. This means that g = f k for some k ∈ K[x]. But now by Remark 8.4 we
can find ab ∈ K (with a and b coprime) such that ab f and ab k are in R[x]. Since the greatest common
divisor of the coefficients of f is 1 this is only possible if b is a unit. But then k = ab−1 ( ab k) ∈ R[x],
and so f | g in R[x].
Remark 8.6. Of course, Proposition 8.5 implies by induction that R[x1 , . . . , xn ] = R[x1 ][x2 ] · · · [xn ] is
a unique factorization domain if R is. In particular, the polynomial ring K[x1 , . . . , xn ] over a field K
is a unique factorization domain. This also shows that there are more unique factorization domains
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Andreas Gathmann
than principal ideal domains: as the ideal (x1 , . . . , xn ) in K[x1 , . . . , xn ] cannot be generated by fewer
than n elements by Exercise 1.9, this polynomial ring is a principal ideal domain only for n = 1.
Example 8.7 (Geometric interpretation of prime and irreducible elements). Consider the coordinate
ring R = R[x, y]/(x2 + y2 − 1) of the unit circle X = V (x2 + y2 − 1) ⊂ A2R . Note that x2 + y2 − 1 is
obviously irreducible (it cannot be written as a product of two linear polynomials since otherwise
X would have to be a union of two lines), and hence prime by Proposition 8.1 since R[x, y] is a
unique factorization domain by Remark 8.6. So (x2 + y2 − 1) is a prime ideal by Example 2.6 (a),
and consequently R is an integral domain by Lemma 2.3 (a).
We are going to show that R is not a unique factorization domain. In fact, we will
prove — and interpret geometrically — that x ∈ R is irreducible, but not prime.
Note that the zero locus V (x) of x in X consists of the two points a = (0, 1) and
b = (0, −1) shown in the picture on the right. In particular, x is neither 0 in R
(otherwise V (x) would be X) nor a unit (otherwise V (x) would be empty).
(a) x is not prime: Geometrically, by Remark 2.7 (b) this is just the statement that V (x) is not an irreducible variety since it consists of two
points.
a
X
b
Algebraically, x divides x2 = (1 + y)(1 − y) in R, but it does not divide 1 ± y: if e. g. we had
x | 1 + y this would mean 1 + y = gx + h(x2 + y2 − 1) for some g, h ∈ R[x, y], but plugging in
the point a would then give the contradiction 2 = 0.
(b) x is irreducible: otherwise we would have x = f g for two non-units f and g in R.
Intuitively, as the function x vanishes on X exactly at the two points a and b with multiplicity
1, this would mean that one of the two factors, say f , would have to vanish exactly at a
with multiplicity 1, and the other g exactly at b. But this would mean that the curve V ( f ) in
A2R meets the circle X exactly at one point a with multiplicity 1. This seems geometrically
impossible since the circle X has an outside and an inside, so if V ( f ) crosses the circle and
goes from the outside to the inside, it has to cross it again somewhere (as the dashed line in
the picture above) since it cannot end in the interior of the circle.
To give an exact argument for this requires a bit more work. Note that every element h ∈ R
has a unique representative of the form h0 + x h1 ∈ R[x, y] with h0 , h1 ∈ R[y]. We define a
“norm” map
N : R → R[y], h 7→ h20 + (y2 − 1) h21
which can also be thought of as taking the unique representative of h(x, y) · h(−x, y) not
containing x. In particular, N is multiplicative (which can of course also be checked directly).
Hence we have
(y + 1)(y − 1) = N(x) = N( f ) N(g).
As R[y] is a unique factorization domain, there are now two possibilities (up to symmetry in
f and g):
• N( f ) is constant: Then f02 + (y2 − 1) f12 is constant. But the leading coefficients of
both f02 and (y2 − 1) f12 are non-negative and thus cannot cancel in the sum, and hence
we must have that f0 is constant and f1 = 0. But then f is a unit in R, which we
excluded.
• N( f ) = a (y − 1) for some a ∈ R\{0}: Then f02 + (y2 − 1) f12 = a (y − 1), and so we
have y − 1 | f0 . So we can write f0 = (y − 1) f00 for some polynomial f00 ∈ R[y] and
obtain (y − 1) f00 2 + (y + 1) f12 = a. This is again a contradiction, since the left hand
side must have a positive non-constant leading term.
Altogether, this contradiction shows that x is in fact irreducible.
Exercise 8.8. In contrast to Example 8.7, show that the following rings are unique factorization
domains:
8.
Prime Factorization and Primary Decompositions
73
(a) the coordinate rings R[x, y]/(y − x2 ) and R[x, y]/(xy − 1) of the standard parabola and hyperbola in A2R , respectively;
(b) C[x, y]/(x2 + y2 − 1).
We will also see in Proposition 12.14 “(e) ⇒ (c)” that the localization of R[x, y]/(x2 + y2 − 1) at the
maximal ideal corresponding to one of the points a and b in Example 8.7 is a unique factorization
domain. For the moment however we just note that the unique factorization property easily breaks
down, and in addition is only defined for integral domains — so let us now study how the concept
of prime factorization can be generalized to a bigger class of rings.
To see the idea how this can be done, let us consider Example 8.7 again. The function x ∈ R was not
prime since its zero locus V (x) was a union of two points. We could not decompose x as a product
of two functions vanishing at only one of the points each, but we can certainly decompose the ideal
(x) into two maximal (and hence prime) ideals as
(x) = I(a) ∩ I(b) = (x, y − 1) ∩ (x, y + 1),
which by Remark 1.12 is just the algebraic version of saying that V (x) is the union of the two points
a and b. So instead of elements we should rather decompose ideals of R, in the sense that we write
them as intersections of “easier” ideals. (Note that in the above example we could also have taken
the product of the two ideals instead of the intersection, but for general rings intersections turn out to
be better-behaved, in particular under the presence of zero-divisors. Decompositions into products
of maximal or prime ideals will be studied in Chapter 13, see e. g. Proposition 13.7 (b) and Example
13.12.)
To see what these “easier” ideals should be, consider the simple case of a principal ideal domain
R: any non-zero ideal I E R can be written as I = (pk11 · · · · · pknn ) for some distinct prime elements
p1 , . . . , pn ∈ R and k1 , . . . , kn ∈ N>0 by Example 8.3 (a), and the “best possible” decomposition of
this ideal as an intersection of easier ideals is
I = (p1 )k1 ∩ · · · ∩ (pn )kn .
So it seems that we are looking for decompositions of ideals as intersections of powers of prime
ideals. Actually, whereas this is the correct notion for principal ideal domains, we need a slight
variant of prime powers for the case of general rings:
Definition 8.9 (Primary ideals). Let R be a ring. An ideal Q E R with Q 6= R is called primary if for
n
all a, b ∈ R with
√ ab ∈ Q we have a ∈ Q or b ∈ Q for some n ∈ N (which is obviously equivalent to
a ∈ Q or b ∈ Q).
Example 8.10 (Primary ideals = powers of prime ideals in principal ideal domains). In a principal
ideal domain R, the primary ideals are in fact exactly the ideals of the form (pn ) for a prime element
p ∈ R and n ∈ N>0 :
• The ideal (pn ) is primary: if ab ∈ (pn ) then ab = cpn for some c ∈ R. But now the n factors
of p are either all contained in a (in which case a ∈ (pn )), or at least one of them is contained
in b (in which case bn ∈ (pn )).
• Conversely, let I = (pk11 · · · · · pknn ) be any primary ideal, where p1 , . . . , pn are distinct primes
and k1 , . . . , kn ∈ N>0 . Then we must have n = 1, since otherwise pk11 · (pk22 · · · · · pknn ) ∈ I, but
neither pk11 nor any power of pk22 · · · · · pknn are in I.
Note that if R is only a unique factorization domain the same argument still works for principal ideals
— but not for arbitrary ideals as we will see in Example 8.13 (a).
Remark 8.11. Let R be a ring.
(a) Obviously, every prime ideal in R is primary, but the converse does not hold by Example
8.10.
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Andreas Gathmann
√
(b) However, if Q E R is primary√then P = Q is prime: if ab ∈ P√then (ab)n ∈ Q for some
n ∈ N. Hence an ∈ Q or bn ∈ Q, which means that a or b lie in Q = P. In fact, P is then
the smallest prime ideal containing Q by Lemma 2.21.
√
If we want to specify the underlying prime ideal P = Q of a primary ideal Q we often say
that Q is P-primary.
(c) The condition of an ideal Q E R with Q 6= R being primary can also be expressed in terms of
the quotient ring R/Q: obviously, Definition 8.9 translates into the requirement that a b = 0
implies a = 0 or b n = 0 for some n ∈ N. This means for every element b that b n = 0 for
some n ∈ N if there is an exists a 6= 0 with a b = 0. So Q is primary if and only if every
zero-divisor of R/Q is nilpotent.
As in Corollary 2.4 this means that primary ideals are preserved under taking quotients, i. e.
for an ideal I ⊂ Q we have that Q is primary in R if and only if Q/I is primary in R/I.
To obtain more examples of primary ideals, we need the following lemma.
It gives a particularly
√
easy criterion to detect a primary ideal Q if its underlying prime ideal Q is maximal.
Lemma 8.12 (Primary ideals over maximal ideals). Let P be a maximal ideal in a ring R. If an ideal
Q E R satisfies one of the following conditions:
√
(a) Q = P;
(b) Pn ⊂ Q ⊂ P for some n ∈ N>0 ;
then Q is P-primary.
Proof.
p
(a) The given condition means that in R/Q the nilradical (0) is equal to P/Q, hence maximal
by Corollary 2.4. Exercise 2.25 then implies that every element of R/Q is either a unit or
nilpotent. Therefore every zero-divisor of R/Q (which is never a unit) is nilpotent, and so Q
is primary by Remark 8.11 (c).
√
√
√
√
(b) Taking
radicals in the given inclusions yields P = Pn ⊂ Q ⊂ P, and hence we get
√
√
Q = P = P. So the result then follows from (a).
Example 8.13 (Primary ideals 6= powers of prime ideals). In general, primary ideals and powers of
prime ideals are different objects:
2
(a) Primary
√ ideals need not be powers of prime ideals: let Q = (x , y) and P = (x, y) in R[x, y].
Then Q = P is maximal by Example 2.6 (c), and so Q is P-primary by Lemma 8.12 (a).
However, (x2 , xy, y2 ) = P2 ( Q ( P = (x, y), hence Q is not a power of P.
(b) Powers of prime ideals need not be primary: Let R = R[x, y, z]/(xy − z2 ) and P = (x, z) E R.
Then P is prime by Lemma 2.3 (a) since R/P ∼
= R[y] is an integral domain. But the power
P2 = (x2 , x z, z2 ) is not primary as x y = z2 ∈ P2 , but neither is x in P2 nor any power of y.
√
However, if R is Noetherian and Q primary with Q = P, then Q always contains a power of the
prime ideal P by Exercise 7.22 (b).
Remark 8.14. Note that the condition of Definition 8.9 for a primary ideal Q is not symmetric in the
two factors a and b, i. e. it does not say that ab ∈ Q implies that one of the two factors a and b lie in
√
Q. In fact, Example 8.13 (b) shows that this latter condition is not equivalent to Q being primary
as it is always satisfied by powers of prime ideals: if P is prime and√ab ∈ P√n for some n ∈ N>0 then
we also have ab ∈ P, hence a ∈ P or b ∈ P, and so a or b lie in P = P = Pn .
Let us now prove that every ideal in a Noetherian ring can be decomposed as an intersection of
primary ideals.
Definition 8.15 (Primary decompositions). Let I be an ideal in a ring R. A primary decomposition
of I is a finite collection {Q1 , . . . , Qn } of primary ideals such that I = Q1 ∩ · · · ∩ Qn .
8.
Prime Factorization and Primary Decompositions
75
Proposition 8.16 (Existence of primary decompositions). In a Noetherian ring every ideal has a
primary decomposition.
Proof. Assume for a contradiction that R is a Noetherian ring that has an ideal without primary
decomposition. By Lemma 7.4 (a) there is then an ideal I E R which is maximal among all ideals in
R without a primary decomposition. In the quotient ring S := R/I the zero ideal I/I is then the only
one without a primary decomposition, since by Remark 8.11 (c) contraction and extension by the
quotient map give a one-to-one correspondence between primary decompositions of an ideal J ⊃ I
in R and primary decompositions of J/I in R/I.
In particular, the zero ideal (0) E S is not primary itself, and so there are a, b ∈ S with ab = 0, but
a 6= 0 and bn 6= 0 for all n ∈ N. Now as R is Noetherian, so is S by Remark 7.8 (b), and hence the
chain of ideals
ann(b) ⊂ ann(b2 ) ⊂ ann(b3 ) ⊂ · · ·
becomes stationary, i. e. there is an n ∈ N such that ann(bn ) = ann(bn+1 ).
Note that (a) 6= 0 and (bn ) 6= 0 by our choice of a and b. In particular, these two ideals have a
primary decomposition. Taking the primary ideals of these two decompositions together, we then
obtain a primary decomposition of (a) ∩ (bn ) as well. But (a) ∩ (bn ) = 0: if x ∈ (a) ∩ (bn ) then
x = ca and x = dbn for some c, d ∈ S. As ab = 0 we then have 0 = cab = xb = dbn+1 , hence
d ∈ ann(bn+1 ) = ann(bn ), which means that x = dbn = 0. This is a contradiction, since the zero
ideal in S does not have a primary decomposition by assumption.
Example 8.17.
(a) In a unique factorization domain R every principal ideal I = (pk11 · · · · · pknn ) has a primary
decomposition
I = (p1 )k1 ∩ · · · ∩ (pn )kn
by Example 8.10 (where p1 , . . . , pn are distinct prime elements and k1 , . . . , kn ∈ N>0 ).
(b) Geometrically, if I = Q1 ∩ · · · ∩ Qn is a primary decomposition of an ideal I in the coordinate
ring of a variety X, we have
V (I) = V (Q1 ) ∪ · · · ∪V (Qn ) = V (P1 ) ∪ · · · ∪V (Pn )
√
by Remark 1.12, where Pi = Qi for i = 1, . . . , n. So by Remark 2.7 we have decomposed
the subvariety Y = V (I) as a union of irreducible subvarieties V (Pi ). As coordinate rings of
varieties are always Noetherian by Remark 7.15, Proposition 8.16 asserts that such a decomposition of a (sub-)variety into finitely many irreducible subvarieties is always possible.
However, the primary decomposition of an ideal I E A(X) contains more information that is
not captured in its zero locus V (I) alone: we do not only get subvarieties whose union is
V (I), but also (primary) ideals whose zero loci are these subvarieties. These primary ideals
can be thought of as containing additional “multiplicity information”: for example, the zero
locus of the ideal I = ((x − a1 )k1 · · · · · (x − an )kn ) in R[x] is the subset {a1 , . . . , an } of R —
but the ideal also associates to each point ai a multiplicity ki , and the primary decomposition
I = ((x − a1 )k1 ) ∩ · · · ∩ ((x − an )kn )
as in (a) remembers these multiplicities.
In fact, in higher dimensions the additional information at each subvariety is more complicated than just a multiplicity. We will not study this here in detail, however we will see an
instance of this in Example 8.23.
Having proven that primary decompositions always exist in Noetherian rings, we now want to see in
the rest of this chapter to what extent these decompositions are unique. However, with our current
definitions it is quite obvious that they are far from being unique, since they can be changed in two
simple ways:
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Andreas Gathmann
Example 8.18 (Non-uniqueness of primary decompositions).
(a) We can always add “superfluous ideals” to a primary decomposition, i. e. primary ideals that
are already contained in the intersection of the others. For example, (x2 ) and (x) ∩ (x2 ) in
R[x] are two primary decompositions of the same ideal (x2 ) by Example 8.10.
(b) In a given primary decomposition we might have several primary ideals with the same underlying radical, such as in
(x2 , xy, y2 ) = (x2 , y) ∩ (x, y2 )
(∗)
in R[x, y]. Note that this equation holds since the ideals (x2 , y), (x, y2 ), and (x2 , xy, y2 ) contain
exactly the polynomials without the monomials 1 and x, 1 and y, and without any constant
or linear terms, respectively. Moreover, all three ideals have the radical P = (x, y), and hence
they are all P-primary by Lemma 8.12 (a). So (∗) are two different primary decompositions
of the same ideal, in which none of the ideals is superfluous as in (a).
By a slight refinement of the definitions it is actually easy to remove these two ambiguities from
primary decompositions. To do this, we need a lemma first.
Lemma 8.19 (Intersections of primary ideals). Let P be a prime ideal in a ring R. If Q1 and Q2 are
two P-primary ideals in R, then Q1 ∩ Q2 is P-primary as well.
√
√
√
Proof. First of all, by Lemma 1.7 (b) we have Q1 ∩ Q2 = Q1 ∩ Q2 = P ∩ P = P. Now let
ab ∈ Q1 ∩ Q2 , i. e. ab ∈ Q1 and ab ∈ Q2 . As Q1 and Q2 are P-primary we
√ know that a ∈ Q1 or b ∈ P,
as well as a ∈ Q2 or b ∈ P. This is the same as a ∈ Q1 ∩ Q2 or b ∈ P = Q1 ∩ Q2 . Hence Q1 ∩ Q2 is
P-primary.
Definition 8.20 (Minimal primary decompositions).
Let {Q1 , . . . , Qn } be a primary decomposition
√
of an ideal I in a ring R, and let Pi = Qi for i = 1, . . . , n. Then the decomposition is called minimal
if
(a) none of the ideals is superfluous in the intersection, i. e.
T
j6=i Q j
6⊂ Qi for all i;
(b) Pi 6= Pj for all i, j with i 6= j.
Corollary 8.21 (Existence of minimal primary decompositions). If an ideal in a ring has a primary
decomposition, it also has a minimal one.
In particular, in a Noetherian ring every ideal has a minimal primary decomposition.
Proof. Starting from any primary decomposition, leave out superfluous ideals, and replace ideals
with the same radical by their intersection, which is again primary with the same radical by Lemma
8.19.
The additional statement follows in combination with Proposition 8.16.
Exercise 8.22. Find a minimal primary decomposition of . . .
(a) the ideal I = (x2 ) in the ring R = R[x, y]/(x2 + y2 − 1) (see Example 8.7);
√
(b) the ideal I = (6) in the ring R = Z[ 5 i] (see Example 8.3 (b));
(c) the ideal I = (x, y) · (y, z) in the ring R[x, y, z].
As a consequence of Corollary 8.21, one is usually only interested in minimal primary decompositions. However, even then the decompositions will in general not be unique, as the following
example shows.
Example 8.23 (Non-uniqueness of minimal primary decompositions). Let us consider the ideal
I = (y) · (x, y) = (xy, y2 ) in R[x, y]. Geometrically, the zero locus of this ideal is just the horizontal
axis V (I) = V (y), which is already irreducible. However, I is not primary since yx ∈ I, but y ∈
/ I and
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Prime Factorization and Primary Decompositions
77
xn ∈
/ I for all n ∈ N. Hence, I is not its own minimal primary decomposition. However, we claim
that
I = Q1 ∩ Q2 = (y) ∩ (x2 , xy, y2 ) and I = Q1 ∩ Q02 = (y) ∩ (x, y2 )
are two different minimal primary decompositions of I. In fact, both equations can be checked in the
same way as in Example 8.18 (b) (the ideal I contains exactly the polynomials with no monomial
1, y, or xn for n ∈ N). Moreover, Q1 is primary since it is prime, and Q2 and Q02 are primary by
Lemma 8.12 (a) since both of them have the same maximal radical P2 = (x, y). Finally, it is clear that
in both decompositions none of the ideals is superfluous, and that the radicals of the two ideals are
different — namely P1 = (y) with zero locus X1 = V (P1 ) = R × {0} and P2 = (x, y) with zero locus
X2 = V (P2 ) = {(0, 0)}, respectively.
So geometrically, even our minimal primary decompositions contain a so-called embedded component, i. e. a subvariety X2 contained in another subvariety X1 of the decomposition, so that it is not
visible in the zero locus of I. The other component X1 is usually called an isolated component. The
corresponding algebraic statement is that P2 contains another prime ideal P1 occurring as a radical
in the decomposition; we will also say that P2 is an embedded and P1 an isolated prime ideal (see
Definition 8.25 and Example 8.28).
The intuitive reason why this embedded component occurs is that X2 has a
higher “multiplicity” in I than X1 (in a sense that we do not want to make
precise here). We can indicate this as in the picture on the right by a “fat
point” X2 on a “thin line” X1 .
X2
X1
In any case, we conclude that even minimal primary decompositions of an ideal are not unique.
However, this non-uniqueness is very subtle: we will now show that it can only occur in the primary ideals of embedded components. More precisely, we will prove that in a minimal primary
decomposition:
(a) the underlying prime ideals of all primary ideals are uniquely determined (see Proposition
8.27); and
(b) the primary ideals corresponding to all isolated components are uniquely determined (see
Proposition 8.34).
So in our example above, P1 , P2 , and Q1 are uniquely determined, and only the primary ideal corresponding to P2 can depend on the decomposition.
To prove the first statement (a), we give an alternative way to reconstruct all underlying prime ideals
of a minimal primary decomposition (the so-called associated prime ideals) without knowing the
decomposition at all.
Lemma 8.24. Let Q be a P-primary ideal in a ring R. Then for any a ∈ R we have
(
p
R if a ∈ Q,
Q:a =
P if a ∈
/ Q.
Proof. If a ∈ Q then clearly Q : a = R, and thus
√
Q : a = R as well.
Now let a ∈
/ Q. Then for any b ∈ Q : a we have ab ∈√Q, and so b ∈ P since Q is P-primary. Hence
Q ⊂ Q : a ⊂ P, and by taking radicals we obtain P ⊂ Q : a ⊂ P as desired.
Definition 8.25 (Associated, isolated, and embedded prime ideals). Let I be an ideal in a ring R.
√
(a) An associated prime ideal of I is a prime ideal that can be written as I : a for some a ∈ R.
We denote the set of these associated primes by Ass(I).
(b) The minimal elements of Ass(I) are called isolated prime ideals of I, the other ones embedded prime ideals of I.
√
Remark 8.26. For an ideal I of a ring R, note that not every ideal that can be written as I : a for
some a ∈ R is prime. By definition, Ass(I) contains only the prime ideals of this form.
78
Andreas Gathmann
Proposition 8.27 (First Uniqueness Theorem for primary decompositions).
Let Q1 , . . . , Qn form a
√
minimal primary decomposition for an ideal I in a ring R, and let Pi = Qi for i = 1, . . . , n.
Then {P1 , . . . , Pn } = Ass(I). In particular, the number of components in a minimal primary decomposition and their radicals do not depend on the chosen decomposition.
Proof.
“⊂”: Let i ∈ {1, . . . , n}, we will show that Pi ∈ Ass(I). As the given decomposition is minimal,
T
we can find a ∈ j6=i Q j with a ∈
/ Qi . Then
p
√
I : a = Q1 : a ∩ · · · ∩ Qn : a
p
p
= Q1 : a ∩ · · · ∩ Qn : a (Lemma 1.7 (b))
= Pi
and so Pi ∈ Ass(I).
“⊃”: Let P ∈ Ass(I), so P =
(Lemma 8.24),
√
I : a for some a ∈ R. Then as above we have
p
p
√
P = I : a = Q1 : a ∩ · · · ∩ Qn : a,
√
and thus P ⊃ Qi : a for some
√ i by Exercise 2.10
√(a) since P is prime. But of course the above
equation also implies P ⊂ Qi : a, and so P = Qi : a. Now by Lemma 8.24 this radical can
only be Pi or R, and since P is prime we conclude that we must have P = Pi .
Example 8.28. In the situation of Example 8.23, Proposition 8.27 states that the associated prime
ideals of I are P1 and P2 since we have found a minimal primary decomposition of I with these underlying prime ideals. It is then obvious by Definition 8.25 that P1 is an isolated and P2 an embedded
prime of I.
Exercise 8.29. Let I be an ideal in a√ring R. In Definition 8.25 we have introduced Ass(I) as the set
of prime ideals that are of the form I : a for some a ∈ R. If R is Noetherian, prove that we do not
have to take radicals, i. e. that Ass(I) is also equal to the set of all prime ideals that are of the form
I : a for some a ∈ R.
Corollary 8.30 (Isolated prime ideals = minimal prime ideals). Let I be an ideal in a Noetherian
ring R. Then the isolated prime ideals of I are exactly the minimal prime ideals over I as in Exercise
2.23, i. e. the prime ideals P ⊃ I such that there is no prime ideal Q with I ⊂ Q ( P.
In particular, in a Noetherian ring there are only finitely many minimal prime ideals over any given
ideal.
Proof. As R is Noetherian, there is √
a minimal primary decomposition I = Q1 ∩ · · · ∩ Qn of I by
Corollary 8.21. As usual we set Pi = Qi for all i, so that Ass(I) = {P1 , . . . , Pn } by Proposition 8.27.
Note that if P ⊃ I is any prime ideal, then
√ P ⊃ Q1 ∩ · · · ∩ Qn , hence P ⊃ Qi for some i by Exercise
2.10 (a), and so by taking radicals P ⊃ Qi = Pi . With this we now show both implications stated
in the corollary:
• Let Pi ∈ Ass(I) be an isolated prime ideal of I. If P is any prime ideal with I ⊂ P ⊂ Pi then
by what we have just said Pj ⊂ P ⊂ Pi for some j. But as Pi is isolated we must have equality,
and so P = Pi . Hence Pi is minimal over I.
• Now let P be a minimal prime over I. By the above I ⊂ Qi ⊂ Pi ⊂ P for some i. As P is
minimal over I this means that P = Pi is an associated prime, and hence also an isolated
prime of I.
Remark 8.31. In particular, Corollary 8.30 states that the isolated prime ideals of an ideal I in a coordinate ring of a variety A(X) correspond exactly to the maximal subvarieties, i. e. to the irreducible
components of V (I) — as already motivated in Example 8.23.
Exercise 8.32. Let R be a Noetherian integral domain. Show:
8.
Prime Factorization and Primary Decompositions
79
(a) R is a unique factorization domain if and only if every minimal prime ideal over a principal
ideal is itself principal.
(b) If R is a unique factorization domain then every minimal non-zero prime ideal of R is principal.
Finally, to prove the second uniqueness statement (b) of Example 8.23 the idea is to use localization
at an isolated prime ideal to remove from I all components that do not belong to this prime ideal.
Lemma 8.33. Let S be a multiplicatively closed subset in a ring R, and let Q be a P-primary ideal
in R. Then with respect to the ring homomorphism ϕ : R → S−1 R, a 7→ a1 we have
(
R if S ∩ P 6= 0,
/
e c
(Q ) =
Q if S ∩ P = 0.
/
√
Proof. If S ∩ P 6= 0/ there is an element s ∈ S with s ∈ P = Q, and thus sn ∈ Q for some n ∈ N. So
n
1
s
−1
e
e
−1
e c
1 = sn ∈ S Q = Q by Example 6.18. Hence Q = S R, and therefore (Q ) = R.
On the other hand, assume now that S ∩ P = 0.
/ By Exercise 1.19 (a) it suffices so prove (Qe )c ⊂ Q.
q
a
a
e
c
e
If a ∈ (Q ) we have 1 ∈ Q , and so 1 = s for some q ∈ Q and s ∈ S by Proposition 6.7 (a). Hence
u(q − as) = 0 for some u ∈ S, which implies that a · us = uq ∈ Q. As Q is P-primary it follows that
a ∈ Q or us ∈ P. But us ∈ S, so us ∈
/ P since S ∩ P = 0,
/ and we conclude that a ∈ Q.
Proposition 8.34 (Second Uniqueness Theorem for primary decompositions).
Let Q1 , . . . , Qn form
√
a minimal primary decomposition for an ideal I in a ring R, and let Pi = Qi for i = 1, . . . , n.
If i ∈ {1, . . . , n} such that Pi is minimal over I, then (I e )c = Qi , where contraction and extension are
taken with respect to the canonical localization map R → RPi . In particular, in a minimal primary
decomposition the primary components corresponding to minimal prime ideals do not depend on the
chosen decomposition.
Proof. Localizing the equation I = Q1 ∩ · · · ∩ Qn at S gives S−1 I = S−1 Q1 ∩ · · · ∩ S−1 Qn by Exercise
6.24 (b), hence I e = Qe1 ∩ · · · ∩ Qen by Example 6.18, and so (I e )c = (Qe1 )c ∩ · · · ∩ (Qen )c by Exercise
1.19 (d).
Now let Pi be minimal over I, and set S = R\Pi . Then S ∩ Pi = 0,
/ whereas S ∩ Pj 6= 0/ for all j 6= i
since Pj 6⊂ Pi . So applying Lemma 8.33 gives (I e )c = (Qei )c = Qi as desired.
15